Efficient Physical Layer Preamble Format

ABSTRACT

A method for generating a preamble of a data unit for transmission via a communication channel includes generating a first field of the preamble using one of a first sequence or a second sequence, such that the first sequence and the second sequence are complementary sequences such that a sum of out-of-phase aperiodic autocorrelation coefficients of the first sequence and the second sequence is zero; generating, using the other one of the first sequence or the second sequence, an indicator of a start of a second field of the preamble, the second field associated with channel estimation information, such that the indicator of the start of the second field immediately follows the first field; and generating the second field of the preamble.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplications Nos. 61/053,526 filed May 15, 2008 and 61/078,925 filedJul. 8, 2008, both entitled “PHY Preamble Format for 60 GHz WidebandWireless Communication Systems,” the disclosure of each of which ishereby expressly incorporated herein by reference; to U.S. ProvisionalPatent Applications Nos. 61/080,514 filed Jul, 14, 2008, 61/084,133filed Jul. 28, 2008, 61/084,776 filed Jul. 30, 2008, 61/085,763 filedAug. 1, 2008, 61/090,058 filed Aug. 19, 2008, 61/091,885 filed Aug. 26,2008, 61/098,128 filed Sep. 18, 2008, 61/098,970 filed Sep. 22, 2008,and 61/100,112 filed Sep. 25, 2008, each of which is entitled “ShortenedPHY Preamble Format for 60 GHz Wideband Wireless Communication Systems,”and the disclosure of each of which is also hereby expresslyincorporated herein by reference; and to U.S. Provisional PatentApplications Nos. 61/099,790 filed Sep. 24, 2008, and 61/102,152 filedOct. 2, 2008, each of which is entitled “Enhanced Channel EstimationFormat and Packet Indication for mm Wave Applications,” the disclosureof each of which is also hereby expressly incorporated herein byreference.

This application is also related to the following commonly-owned,co-pending patent applications: U.S. patent application Ser. No. ______(Attorney Docket No. MP2693.D1), entitled “Efficient PHY PreambleFormat,” U.S. patent application Ser. No. ______ (Attorney Docket No.MP2693.D2), entitled “Efficient PHY Preamble Format,” and U.S. patentapplication Ser. No. ______ (Attorney Docket No. MP2693.D3), entitled“Apparatus for Generating Spreading Sequences and DeterminingCorrelation,” all filed on the same day as the present application, andall hereby expressly incorporated by reference herein.

FIELD OF TECHNOLOGY

The present disclosure relates generally to communication systems and,more particularly, to information formats for exchanging information viacommunication channels.

BACKGROUND

An ever-increasing number of relatively inexpensive, low power wirelessdata communication services, networks and devices have been madeavailable over the past number of years, promising near wire speedtransmission and reliability. Various wireless technology is describedin detail in several IEEE standards documents, including for example,the IEEE Standard 802.11b (1999) and its updates and amendments, as wellas the IEEE 802.15.3 Draft Standard (2003) and the IEEE 802.15.3c DraftD0.0 Standard, all of which are collectively incorporated herein fullyby reference.

As one example, a type of a wireless network known as a wirelesspersonal area network (WPAN) involves the interconnection of devicesthat are typically, but not necessarily, physically located closertogether than wireless local area networks (WLANs) such as WLANs thatconform to the IEEE Standard 802.11a. Recently, the interest and demandfor particularly high data rates (e.g., in excess of 1 Gbps) in suchnetworks has significantly increased. One approach to realizing highdata rates in a WPAN is to use hundreds of MHz, or even several GHz, ofbandwidth. For example, the unlicensed 60 GHz band provides one suchpossible range of operation.

In general, transmission systems compliant with the IEEE 802 standardssupport one or both of a Single Carrier (SC) mode of operation or anOrthogonal Frequency Division Multiplexing (OFDM) mode of operation toachieve higher data transmission rates. For example, a simple, low-powerhandheld device may operate only in the SC mode, a more complex devicethat supports a longer range of operation may operate only in the OFDMmode, and some dual-mode devices may switch between SC and OFDM modes.

Generally speaking, the use of OFDM divides the overall system bandwidthinto a number of frequency sub-bands or channels, with each frequencysub-band being associated with a respective subcarrier upon which datamay be modulated. Thus, each frequency sub-band of the OFDM system maybe viewed as an independent transmission channel within which to senddata, thereby increasing the overall throughput or transmission rate ofthe communication system. During operation, a transmitter operating inthe OFDM mode may encode the information bits (which may include errorcorrection encoding and interleaving), spread the encoded bits using acertain spreading sequence, map the encoded bits to symbols of a 64quadrature amplitude modulation (QAM) multi-carrier constellation, forexample, and transmit the modulated and upconverted signals afterappropriate power amplification to one or more receivers, resulting in arelatively high-speed time domain signal with a large peak-to-averageratio (PAR).

Likewise, the receivers generally include a radio frequency (RF)receiving unit that performs correlation and demodulation to recover thetransmitted symbols, and these symbols are then processed in a Viterbidecoder to estimate or determine the most likely identity of thetransmitted symbol. The recovered and recognized stream of symbols isthen decoded, which may include deinterleaving and error correctionusing any of a number of known error correction techniques, to produce aset of recovered signals corresponding to the original signalstransmitted by the transmitter.

Specifically with respect to wideband wireless communication systemsthat operate in the 60 GHz band, the IEEE 802.15.3c Draft D0.0 Standard(“the Proposed Standard”) proposes that each packet transmitted via acommunication channel include a preamble to provide synchronization andtraining information; a header to provide the basic parameters of thephysical layer (PHY) such as length of the payload, modulation andcoding method, etc.; and a payload portion. A preamble consistent withthe Proposed Standard includes a synchronization field (SYNC) toindicate the beginning of a block of transmitted information for signaldetection, a start frame delimiter (SFD) field to signal the beginningof the actual frame, and a channel estimation sequence (CES). Thesefields can carry information for receiver algorithms related toautomatic gain control (AGC) setting, antenna diversity selection orphase array setting, timing acquisition, coarse frequency offsetestimation, channel estimation, etc. For each of the SC and OFDM modesof operation, the Proposed Standard specifies a unique PHY preamblestructure, i.e., particular lengths of SYNC, SFD, and CES fields as wellas spreading sequences and cover codes (sequences of symbols transmittedusing the corresponding spreading sequences) for each PHY preamblefield.

In addition to being associated with separate structures in SC and OFDMmodes, the frame of a PHY preamble consistent with the Proposed Standardfails to address other potential problems such as low sensitivity, forexample. In particular, the receiver of a PHY preamble may use either acoherent or a noncoherent method to detect the beginning of the SFDfield and accordingly establish frame timing. In general, the coherentmethod requires channel estimation based on the signal in the SYNCfield, which may be performed in an adaptive fashion. However, the SYNCsignal may be too short for channel estimation adaptation to converge toa reliable value. On the other hand, the noncoherent method is not basedon channel estimation and is generally simpler. However, the noncoherentmethod is associated with low sensitivity, i.e., frame timing accuracymay be poor at low signal-to-noise (SNR) levels. Because frame timing iscritical to receiving the entire packet, low sensitivity in frame timingdetection significantly limits overall performance.

SUMMARY

In one embodiment, a method for generating a preamble of a data unit fortransmission via a communication channel includes generating a firstfield of the preamble using one of a first sequence or a secondsequence, such that the first sequence and the second sequence arecomplementary sequences such that a sum of out-of-phase aperiodicautocorrelation coefficients of the first sequence and the secondsequence is zero; generating, using the other one of the first sequenceor the second sequence, an indicator of a start of a second field of thepreamble, the second field associated with channel estimationinformation, such that the indicator of the start of the second fieldimmediately follows the first field; and generating the second field ofthe preamble.

In various implementations, one or more of the following features may beincluded. The indicator of the start of the second field may occurbefore the start of the second field. The indicator of the start of thesecond field may be a start frame delimiter (SFD). The indicator of thestart of the second field may be included in the second field. The firstsequence and the second sequence may be complementary Golay sequences.Each of the sequence and the second sequence may be a 128-chip Golayspreading sequence associated with a weight vector W and a delay vectorD, such that W =[1 1 −1 1 −1 1 −1], and D includes exactly one of eachnumber 1, 2, 4, 8, 16, 32, and 64. The D vector may be one of [1 2 4 816 32 64], [64 16 32 1 8 2 4], or [64 32 16 8 4 2 1]. The method mayinclude modulating the preamble according to a modulation scheme. Themodulation scheme may comprise Binary Phase-Shift Keying (BPSK). Themodulation scheme may be π/2 BPSK. Each of the first sequence and thesecond sequence may be a sequence of binary chips. The first field maybe associated with providing synchronization. Generating the secondfield of the preamble may include generating a first channel estimationsequence (CES) symbol including the first sequence and the secondsequence augmented by a first set of cover codes, and generating asecond channel estimation sequence (CES) symbol including the firstsequence and the second sequence augmented by a second set of covercodes, such that the first CES symbol and the second CES symbol arecomplementary sequences. The first CES symbol and the second CES symbolmay be complementary Golay sequences. The indicator of the start of thesecond field of the preamble may serve as a cyclic prefix of the firstCES symbol. The method may include generating a cyclic postfix of thefirst CES symbol. A last portion of the first field may serve as acyclic prefix of the first CES symbol. A sequence in a field differentthan the second field may serve as a cyclic prefix of the first CESsymbol. A first portion of the second CES symbol may serve as a cyclicpostfix of the first CES symbol, and a last portion of the first CESsymbol may serve as a cyclic prefix of the second CES symbol. The methodmay include generating a cyclic postfix of the second CES symbol. Themethod may include generating a cyclic prefix of the first CES symbol,generating a cyclic postfix of the first CES symbol, and generating acyclic prefix of the second CES symbol. The method may includegenerating a cyclic postfix of the second CES symbol. A length of eachof the first sequence and the second sequence may be an integer N, and alength of each of the first CES symbol and the second CES symbol may beat least 4N. A beginning portion of the first CES symbol may be the sameas a beginning portion of the second CES symbol, an ending portion ofthe first CES symbol may be the same as an ending portion of the secondCES symbol, generating the second field of the preamble with a firstorder of the first CES symbol and the second CES symbol in the secondfield may indicate a first communication mode, and generating the secondfield of the preamble with a second order of the first CES symbol andthe second CES symbol in the second field may indicate a secondcommunication mode. Generating the first field of the preamble with thefirst sequence may indicate a first communication mode, and generatingthe first field of the preamble with the second sequence may indicate asecond communication mode. The first communication mode may be a singlecarrier mode, and the second communication mode may be an orthogonalfrequency division multiplexing (OFDM) mode. Generating the first fieldmay include applying a first cover code to the first field to indicate afirst value of a communication parameter, and applying a second covercode to the first field to indicate a second value of the parameter. Themethod may include transmitting the preamble.

In another embodiment, a method for processing a preamble of a data unitreceived via a communication channel, such that the preamble includes afirst field and a second field associated with channel estimationinformation, such that an ending portion of the first field includes afirst sequence and a beginning portion of the second field includes asecond sequence complementary to the first sequence, and such that a sumof out-of-phase aperiodic autocorrelation coefficients of the firstsequence and the second sequence is zero, includes generating aplurality of correlation signals using a received signal correspondingto the preamble, such that the plurality of correlation signals includeat least two of 1) a cross-correlation between the received signal andthe first sequence, 2) a cross-correlation between the received signaland the first sequence, and 3) an autocorrelation of the receivedsignal; detecting a beginning of the second field based on the pluralityof correlation signals; and using the detection of the beginning of thesecond field to decode the second field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a communication system including atransmitter and a receiver that may communicate using efficient PHYpreambles;

FIG. 2 depicts block diagrams of a transmitter and a receiver that mayoperate in the system of in FIG. 1;

FIG. 3 is a block diagram of a prior art PHY preamble for the SCcommunication mode;

FIG. 4 is a block diagram of a prior art PHY preamble for the OFDMcommunication mode;

FIG. 5 is a block diagram of an example PHY preamble controller thatgenerates an efficient PHY preamble;

FIG. 6 is a block diagram of an example PHY preamble processor thatprocesses the efficient PHY preamble generated by the PHY preamblecontroller illustrated in FIG. 5;

FIG. 7 depicts several example correlation diagrams of a received signaland a pair of complementary Golay sequences;

FIG. 8 is a block diagram of a general structure of an example efficientPHY preamble including a short training field (STF) and a long trainingfield (LTF);

FIG. 9 is a block diagram of an efficient PHY preamble in which a pairof complementary spreading sequences signal the boundary between twotraining fields;

FIG. 10 is a block diagram of an efficient PHY preamble that omitscyclic postfixes in the long training field;

FIG. 11 is a block diagram of an efficient PHY preamble in which thelast period of the short training field corresponds to the cyclic prefixof the first CES symbol in the long training field;

FIG. 12 is a block diagram of an efficient PHY preamble with four-periodCES symbols, in which a pair of complementary spreading sequences signalthe boundary between two training fields;

FIG. 13 is a block diagram of an efficient PHY preamble with four-periodCES symbols that omits cyclic postfixes in the long training field;

FIG. 14 is a block diagram of an efficient PHY preamble with four-periodCES symbols in which the last period of the short training fieldcorresponds to the cyclic prefix of the first CES symbol in the longtraining field;

FIG. 15 is a block diagram of an efficient PHY preamble with four-periodCES symbols in which the last period of the short training fieldcorresponds to the cyclic prefix of the first CES symbol in the longtraining field, and the first period of the second CES symbolcorresponds to the cyclic postfix of the first CES symbol;

FIG. 16 is a block diagram of an efficient PHY preamble that includes aframe delimiter that corresponds to the cyclic prefix of the first CESsymbol of the long training field;

FIG. 17 is a block diagram of an efficient PHY preamble that includes aframe delimiter that includes a cyclic prefix of the first CES symbol ofthe long training field;

FIG. 18 is a block diagram of an efficient PHY preamble with four-periodCES symbols in which the last period of the short training fieldcorresponds to the cyclic prefix of the first CES symbol in the longtraining field, and the last period of the first CES symbol correspondsto the cyclic prefix of the second CES symbol;

FIG. 19 is a block diagram of the efficient PHY preamble of FIG. 18 inwhich the cyclic postfix of the second CES symbol is omitted;

FIG. 20 is a block diagram of another example of a PHY preamble thatincludes a frame delimiter at the end of STF;

FIG. 21 is a block diagram of the efficient PHY preamble of FIG. 20 inwhich the cyclic postfix of the second CES symbol is omitted;

FIG. 22 is a block diagram of the efficient PHY preamble of FIG. 16 thatuses other CES symbols in the long training field;

FIG. 23 is a block diagram of the efficient PHY preamble of FIG. 22 inwhich the cyclic postfix of the second CES symbol is omitted;

FIG. 24 is a block diagram of a PHY preamble format corresponding to thepreamble of FIG. 15 in which the selection of one of two complementarysequences in the STF and LTF fields indicates the selection of a PHYcommunication mode (e.g., SC mode or OFDM mode);

FIG. 25 is a block diagram of a PHY preamble format corresponding to thePHY preamble of FIG. 16 in which the selection of one of twocomplementary spreading sequences in the STF and LTF fields indicatesthe selection of a PHY communication mode (e.g., SC mode or OFDM mode);

FIG. 26 is a block diagram of a PHY preamble format corresponding to thePHY preamble of FIG. 16 in which the selection of one of twocomplementary spreading sequences in the STF field indicates theselection of a PHY communication mode (e.g., SC mode or OFDM mode);

FIG. 27 is a block diagram of a PHY preamble format corresponding to thePHY preamble of FIG. 20 in which the selection of one of twocomplementary spreading sequences in the STF and LTF fields indicatesthe selection of a PHY communication mode (e.g., SC mode or OFDM mode);

FIG. 28 is a block diagram of a PHY preamble format corresponding to thePHY preamble of FIG. 20 in which the selection of one of twocomplementary spreading sequences in the STF field indicates theselection of a PHY communication mode (e.g., SC mode or OFDM mode);

FIG. 29 is a block diagram of an efficient PHY preamble in which an SFDsequence between the STF and LTF fields indicates the selection of a PHYcommunication mode;

FIG. 30 is a block diagram of a PHY preamble format in which a covercode applied to the STF field indicates the selection of a PHYcommunication mode (e.g., SC mode or OFDM mode);

FIG. 31 is a block diagram of another PHY preamble format in which acover code applied to the STF field indicates the selection of a PHYcommunication mode (e.g., SC mode or OFDM mode);

FIG. 32 is a block diagram of an PHY preamble format in which the orderof CES symbols indicates the selection of a PHY communication mode(e.g., SC mode or OFDM mode);

FIG. 33 is a block diagram of another PHY preamble format in which theorder of CES symbols indicates the selection of a PHY communication mode(e.g., SC mode or OFDM mode);

FIG. 34 is a block diagram of several example STF codes in which theselection of a sequence and a cover code indicates the selection of aPHY communication mode (e.g., SC Regular mode, SC Low Rate mode, or OFDMmode);

FIG. 35 is a block diagram of a PHY preamble format in which theselection of one of sequences in the STF, and selection of SFD fieldssignals the selection of a PHY communication mode (e.g., SC Regularmode, SC Low Rate mode, or OFDM mode);

FIG. 36 is a block diagram of a PHY preamble format in which selectionof sequences in the STF field and the pattern in the SFD field indicatesthe selection of a PHY communication mode (e.g., SC Regular mode, SC LowRate mode, or OFDM mode);

FIG. 37 is a block diagram of a PHY preamble format in which cover codesin the STF and SFD fields indicate the selection of a PHY communicationmode (e.g., SC Regular mode, SC Low Rate mode, or OFDM mode);

FIG. 38 is a block diagram of a PHY preamble format in which theselection of a sequence in the STF field and the order of CES trainingsymbols indicates the selection of a PHY communication mode (e.g., SCRegular mode, SC Low Rate mode, or OFDM mode);

FIG. 39 is a block diagram of a PHY preamble in which the length of theLTF field is different for different PHY communication modes;

FIG. 40 is a block diagram of a Golay code generator that generatesefficient Golay sequences for use by devices illustrated in FIG. 1;

FIG. 41 is a block diagram of a correlator for correlating with Golaysequences;

FIG. 42 is a block diagram of a correlator for use with the PHY preambleillustrated in FIG. 15 and that incorporates the correlator of FIG. 41;and

FIG. 43 is a block diagram of a correlator for use with the PHY preambleillustrated in FIG. 18 and that incorporates the correlator of FIG. 41.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an example wireless communication system 10in which devices, such as a transmitting device 12 and a receivingdevice 14, may transmit and receive data packets via a shared wirelesscommunication channel 16. In one embodiment, the devices 12 and 14 maycommunicate according to a communication protocol that utilizes anefficient PHY preamble format as described in greater detail below. Eachof the devices 12 and 14 may be, for example, a mobile station or anon-mobile station equipped with a set of one or more antennas 20-24 and30-34, respectively. Although the wireless communication system 10illustrated in FIG. 1 includes two devices 12, 14, each with threeantennas, the wireless communication system 10 may, of course, includeany number of devices, each equipped with the same or a different numberof antennas (e.g., 1, 2, 3, 4 antennas and so on).

Also, it will be noted that although the wireless communication system10 illustrated in FIG. 1 includes a transmitting device 12 and areceiving device 14, devices in the wireless communication system 10 maygenerally operate in multiple modes (e.g., a transmit mode and a receivemode). Accordingly, in some embodiments, antennas 20-24 and 30-34 maysupport both transmission and reception. Alternatively or additionally,a given device may include separate transmit antennas and separatereceive antennas. It will be also understood that because each of thedevices 12 and 14 may have a single antenna or multiple antennas, thewireless communication system 10 may be a multiple input, multipleoutput (MIMO) system, a multiple input, single output (MISO) system, asingle input, multiple output (SIMO) system, or a single input, singleoutput (SISO) system.

FIG. 2 illustrates, in relevant part, the architectures of thetransmitting device 12 and the receiving device 14. The transmittingdevice 12 may generally convert a sequence of information bits intosignals appropriate for transmission through a wireless channel (e.g.,channel 16 of FIG. 1). More specifically, the transmitting device 12 mayinclude an encoder 52 (e.g., a convolution encoder) that encodesinformation bits, a spreader 54 that converts each encoded bit to asequence of chips, and a modulator 56 that modulates the encoded chipsinto data symbols, which are mapped and converted to signals appropriatefor transmission via one or more transmit antennas 20-24. In general,the modulator 56 may implement any desired modulation techniques basedon one or more of phase shift keying, binary phase-shift keying (BPSK),π/2 BPSK (in which modulation is rotated by π/2 for each symbol or chipso that the maximum phase shift between adjacent symbols/chips isreduced from 180° to 90°), quadrature phase-shift keying (QPSK), π/2QPSK, frequency modulation, amplitude modulation, quadrature amplitudemodulation (QAM), π/2 QAM, on-off keying, minimum-shift keying, Gaussianminimum-shift keying, dual alternative mark inversion (DAMI), etc Insome embodiments, the modulator 56 may include a bit-to-symbol mapper 70that maps encoded bits into symbols, and a symbol-to-stream mapper 72that maps the symbols into multiple parallel streams. If only onetransmit antenna is utilized, the symbol-to-stream mapper 72 may beomitted. Information is transmitted in data units such as packets. Suchdata units typically include a PHY preamble and a PHY payload. Togenerate the PHY preamble, a PHY preamble controller 74 receivescontrols parameters via a control input 76 and sends commands to thespreader 54 and, optionally, the modulator 56, as discussed in moredetail below. The transmitting device 50 may include various additionalmodules that, for purposes of clarity and conciseness, are notillustrated in FIG. 2. For example, the transmitting device 50 mayinclude an interleaver that interleaves the encoded bits to mitigateburst errors. The transmitting device 50 may further include a radiofrequency (RF) front end for performing frequency upconversion, variousfilters, power amplifiers, and so on.

The receiving device 14 may include a pre-processor for space-timeprocessing and equalizer 90 coupled to one or more receive antennas30-34, a PHY preamble processor 92, a demodulator 94, and a decoder 96.The unit 90 may include an equalizer. It will be understood that thereceiving device 14 may also include other components such as filters,analog-to-digital converters, etc. that are omitted from FIG. 2 for thepurposes of clarity and conciseness. The preamble processor 92 mayprocess the received signal in co-operation with the demodulator 94.

In some embodiments, the devices 12 and 14 may communicate using anefficiently formatted PHY preamble that includes the informationincluded in the PHY preamble specified by the IEEE 802.15.3c Draft D0.0Standard, but is of a shorter duration. In some embodiments, the devices12 and 14 convey additional information via the PHY preamble (e.g., PHYcommunication mode, piconet id, etc.). Further, the devices 12 and 14may use a common preamble in different modes of operation (e.g., SC modeand OFDM mode).

To better illustrate the techniques of efficient PHY preambleformatting, prior art formats for SC and OFDM PHY preambles in the IEEE802.15.3c Draft D0.0 Standard, as well as several relevant conceptsrelated to wireless communications, are first discussed with referenceto FIGS. 3 and 4. FIG. 3 is a diagram of a SC mode packet 120 thatincludes an SC PHY preamble 122 having a SYNC field 124, an SFD field126, and a CES field 128; a frame header 130; and a payload with a framecheck sequence (FCS) 132. As indicated above, receivers generally usethe PHY preamble for AGC setting, antenna diversity selection or phasearray setting, timing acquisition, coarse frequency offset estimation,packet and frame synchronization, and channel estimation. The SYNC field132 of the PHY preamble 122 has n periods, each of time T, during eachof which a 128-chip preamble sequence (or “code”) s_(128,m) istransmitted with a positive or negative polarity. In general, the timeof transmission of a preamble sequence may be T. In some embodiments,the length of transmission of a preamble sequence may be less than T.

Depending on the modulation scheme, one, two, four, or other numbers ofdata bits or chips may be mapped to a single symbol. For example, BPSKmodulation maps each binary digit to one of two symbols, while QPSK mapseach pair of binary digits to one of four symbols or constellationpoints. For example, a {0,0} bit tuple may be mapped to a firstconstellation point, a {0,1} bit tuple may be mapped to a secondconstellation point, a {1,0} bit tuple may be mapped to a thirdconstellation point, and a {1,1} bit tuple may be mapped to a fourthconstellation point. Thus, QPSK defines four symbols, and each symbolmay correspond to a particular combination of two binary digits. Othermodulation schemes such as 8-QAM, 16-QAM, 32-QAM, 64-QAM etc., may alsobe utilized.

According to the IEEE 802.15.3c Draft D0.0 Standard, the sequencess_(128,m) are modulated using a π/2 binary phase-shift keying (BPSK)scheme. In the π/2 BPSK scheme, each chip is mapped to one of twosymbols that are 180° apart, and the modulation scheme rotatescounterclockwise by λ/2 each chip. For instance, a first chip in thesequence may be mapped to one of −1 or +1, whereas the next chip in thesequence is mapped to one of +j or −j. The sequences +s_(128,m) and−s_(128,m) may be viewed as binary complements of each other. Also, themodulated signals corresponding to the sequences +s_(128,m) and−s_(128,m) will have a 180° phase shift with respect to each other.

Referring again to FIG. 3, in the notation s_(128,m), the subscript m isan index of one of several available sequences s₁₂₈. In particular,three sequences, s_(128,1), s_(128,2), and s_(128,3), are specified forSC mode, with each of the sequences corresponding to a respectivepiconet id. Once selected, the same spreading sequence is applied inevery period of the fields SYNC 124 and SFD 126, as illustrated in FIG.3.

As used herein, the term “cover code” refers to how a series of preamblesequences are augmented to form a longer sequence. For example, for asequence [+a, −a, +a, −a], where a is a preamble code, the cover codemay be represented as [+1, −1, +1, −1], where −1 may indicate that thebinary complement of the code a is utilized, or that the modulatedsignal corresponding to code −a is phase shifted by 180° with respect tothe modulated signal corresponding to code +a. In this example [+a, −a,+a, −a], the cover code could be represented differently, such as [1, 0,1, 0], where 0 indicates that −a is utilized. In some embodiments, thelonger sequence can be formed by spreading the cover code by one or morepreamble sequences. For instance, the sequence [+a, −a, +a, −a] could begenerated by spreading the cover code [+1, −1, +1, −1] (or [1, 0, 1, 0])by the preamble (or spreading) code a. Similarly, a sequence [+a, −b,−a, +a] could be generated by spreading a cover code [+1, −1, −1, +1](or [1, 0, 0, 1]) by the preamble (or spreading) code a and a preamble(or spreading) code b. In other words, +a could be generated byspreading +1 with a, −b could be generated by spreading −1 with b, andso on. Referring again to FIG. 3, the cover code for the SYNC field 124may be represented as [+1, +1, . . . +1]. The cover code for the SFDfield 126 is a sequence with a length of four. It may vary depending onthe particular preamble that is to be transmitted (e.g., one of twodifferent lengths for the CES field 128, and one of four differentheader spreading factors), but it always begins with −1 (or some otherindicator, such as 0, to indicate that the code −s_(128,m) is to beutilized).

With continued reference to FIG. 3, the CES field 128 includes 256-chipcomplementary Golay sequences a_(256,m) and b_(256,m). To reduce theeffect of inter-symbol interference (ISI), the sequences a_(256,m) andb_(256,m) are preceded by respective cyclic prefixes (a_(pre,m) andb_(pre,m), copies of the last 128 chips of the corresponding sequence)and followed by respective postfixes (a_(pos,m) and b_(pos,m), copies ofthe first 128 chips of the corresponding sequence).

FIG. 4 is a diagram of an OFDM mode packet 150 that includes an OFDM PHYpreamble 152 having a SYNC field 154, an SFD field 156, and a CES field158; a frame header 160; and a payload with a frame check sequence (FCS)162. During each period of the SYNC field 154, a sequence s₅₁₂ istransmitted. Each sequence s₅₁₂ corresponds to four 128-chip preamblesequences a₁₂₈ augmented according to a cover code [c₁, c₂, C₃, C₄].Similarly, he SFD field 156 is a sequence s_(5l2) that corresponds tofour sequences al₂₈, but augmented according to a cover code [d₁, d₂,d₃, d₄]. The CES field 158 comprises 512-chip sequences u₅₁₂ and v₅₁₂and corresponding prefixes (u_(pre) and v_(pre)). The entire packet 150is OFDM modulated.

As can be seen in FIGS. 3 and 4, different preamble formats are utilizedfor SC mode packets and OFDM mode packets. Additionally, the preamblesin the SC mode and the OFDM mode are modulated differently. The presentapplication discloses embodiments of efficient PHY preamble formats andtechniques for formatting and processing such PHY preambles that permita common preamble format to be utilized for both SC mode packets andOFDM mode packets. Further, in some embodiments, efficient PHY preambleformats allow devices to detect boundaries of and/or between preamblefields (e.g., detecting the beginning of the CES field) based on signalcorrelation and without relying on cover codes. Moreover, in someembodiments, the SFD field may be entirely omitted in the PHY preambleif desired. In some embodiments, the efficient PHY preamble of thepresent disclosure includes a short training field (STF) generallyassociated with synchronization information, followed by a long trainingfield (LTF) generally associated with channel estimation information.Still further, in some embodiments, efficient PHY preamble formattingallows certain preamble sequences to fulfill multiple functions, andthereby reduce the overall length of the PHY preamble. For example, apreamble sequence may serve as both a cyclic prefix of a CES symbol anda field delimiter. In some embodiments, an efficient PHY preamble maysignal additional information using CES sequence ordering.

Referring again to FIG. 2, the PHY preamble controller 74 of thetransmitter 12 generally controls the generation of the PHY preamble.Similarly, the PHY preamble processor 92 of the receiver 14 generallyanalyzes the PHY preamble to, for example, identify the location offields and/or field boundaries in the PHY preamble, decode informationencoded in the PHY preamble, etc. The PHY preamble controller 74 isdiscussed in detail with reference to FIG. 5, followed by a discussionof the PHY preamble processor 92 with reference to FIG. 6.

Referring to FIG. 5, the PHY preamble controller 74 may receive variousinput parameters via the control input 76. In one embodiment, the inputparameters may include a PHY mode selector 190 to indicate, for example,one of various SC and OFDM modes of communication; a piconet identifierselector 192 to receive piconet information; a header rate identifier194 to receive, for example, an indication of a rate (e.g., SC (Regular)rate or SC Low Rate Common Mode rate); channel estimation parameters196; etc. In some embodiments, the control input 76 may be coupled to aprocessor such as a PHY processor, other components servicing higherlayers of the communication protocol, etc. The PHY preamble controller74 may include an STF formatter 200 and an LTF formatter 202, each ofwhich may be implemented using hardware, a processor executing machinereadable instructions, or combinations thereof. Each of the formatters200 and 202 is communicatively coupled to at least a signal generator204 and a cover code generator 206. Although FIG. 5 does not depictconnections between the formatters 200-202 and the input signals190-196, the formatters 200-202 may be responsive to at least some ofthe signals of the control input 76.

The signal generator 204 generally receives cover codes and indicationsof when to generate signals using either a chip sequence a or a chipsequence b from the STF formatter 200, the LTF formatter 202 and thecover code generator 206. The chip sequences a and b are complementarysequences. In some embodiments, the signal generator 204 may include amemory device 212, such as RAM, ROM, or another type of memory, to storethe complementary sequences a and b. In other embodiments, the signalgenerator 204 may include a and b sequence generators. In oneembodiments, the signal generator 204 includes a binary selector 210 toselect one of the two complementary sequences a and b for preamblesignal generation. The two complementary sequences a and b havecorrelation properties suitable for detection at a receiving device. Forexample, the complementary spreading sequences a and b may be selectedso that the sum of corresponding out-of-phase aperiodic autocorrelationcoefficients of the sequences a and b is zero. In some embodiments, thecomplementary sequences a and b have a zero or almost-zero periodiccross-correlation. In another aspect, the sequences a and b may haveaperiodic cross-correlation with a narrow main lobe and low-level sidelobes, or aperiodic auto-correlation with a narrow main lobe andlow-level side lobes. In some of these embodiments, the sequences a andb are complementary Golay sequences. Although various lengths of thesequences a and b may be utilized, each of the sequences a and b, insome of the embodiments, has a length of 128-chips.

As is known, complementary Golay sequences may be effectively defined bya weight vector W and a delay vector D that, when applied to a suitablegenerator, produce a pair of complementary sequences. In one embodiment,the weight and delay vectors associated with the sequences a and b aregiven by

W=[1 1 −1 1 −1 1 −1] and  (1)

D=[1 2 4 8 16 32 64].  (2)

The vectors W and D produce the pair of 128-chip Golay sequences

a=ID12E2121D121DEDE2ED1DED1D121DED;  (3)

b=ID12E2121D121DED1D12E212E2EDE212,  (4)

expressed herein in the hexadecimal notation.

In another embodiment, the delay vector D is given by

D=[64 16 32 1 8 2 4].  (5)

Using D with the vectors W given by (1) produces a pair of 128-chipGolay sequences

a=0C950C95A63F59C00C95F36AA63FA63F;  (6)

b=039A039AA93056CF039AFC65A930A930.  (7)

In yet another embodiment, the vector W given by (1) is used with thedelay vector

D=[64 32 16 8 4 2 1]  (8)

to generate

a=4847B747484748B84847B747B7B8B747;  (9)

b=1D12E2121D121DED1D12E212E2EDE212.  (10)

With continued reference to FIG. 5, the cover code generator 206 mayinclude a memory device 220, such as RAM, ROM, or another type ofmemory, to store sets of cover codes. Similarly, the cover codegenerator 206 may include a memory device 222, such as RAM, ROM, oranother type of memory, to store u/v sequences. The cover code generator206 also may include one or more other memory devices to store othersequences that span all or parts of the STF field, all or parts of LTFfield, or both the STF field and the LTF field. In response to commandsfrom the STF formatter 200 and the LTF formatter 202, the cover codegenerator 206 may generate cover codes for a particular PHY preamble.

From the foregoing, it will be appreciated that the PHY preamblecontroller 74 may control the signal generator 204 to generate a PHYpreamble using only one pair of sequences a and b. In general, however,in addition to the sequences a and b, the PHY preamble controller 74 mayalso control the signal generator 204 to utilize other sequences x and yto generate certain parts of the same PHY preamble. Further, the signalgenerator 204 may include a cyclic shifter 230 to generate sequences a′and b′ by cyclically shifting the sequences a and b in response tocertain commands from the formatters 200 and 202.

Now referring to FIG. 6, the PHY preamble processor 92 may include ana/b correlator 250 having an input 252 and two outputs Xa and Xb coupledto a cover code detector 254; a u/v correlator 258; an STF/LTF boundarydetector 260; a channel estimator 262; and a PHY preamble decoder 264.In some embodiments, the channel estimator 262 may be a componentseparate from the PHY preamble processor 92. The PHY preamble decoder264 may provide several output signals including, for example, a PHYmode identifier 270, a piconet identifier 272, and a header rateidentifier 274.

In general, as a correlator (such as the a/b correlator 250) correlatesthe received signal with a sequence s, a peak will occur when thesequence s and a corresponding sequence in the preamble field overlap.When no signal s is present or when the signal-to-noise level is poor,no peak or only small peaks may occur. One technique for measuring peaksin a correlation signal is to generate a peak-to-average measure of thecorrelation signal. Referring specifically to the a/b correlator 250,the signal received via the input 252 may be cross-correlated with thesequence a, cross-correlated with the sequence b, or auto-correlatedwith itself. If desired, the a/b correlator 250 may perform two or allthree of these operations. The a/b correlator 250 may output thecorrelated signals for use by other components of the PHY preambleprocessor 92. Optionally, the a/b correlator 250 may include detectionlogic to determine when the sequence a has been detected and when thesequence b has been detected in the received signal. The a/b correlator250 may output indications of detections of the sequence a and thesequence b. Thus, the output Xa and Xb may be correlation signals, or aand b detection signals.

Next, the cover code detector 254 may determine cover codes associatedwith detected a and b sequences. The cover code detector 254 may supplydetected cover codes and, optionally, detected a and b sequences to thePHY preamble decoder 264 for further processing. For example, if asignal corresponding to [+a, −b, −a, +b] is received, the cover codedetector 254 could send to the PHY preamble decoder 264 an indication ofthe cover code [+1, −1, −1, +1] or, optionally, and indication of thesequence [+a, −b, −a, +b].

The STF/LTF boundary detector 260 may monitor the output of the a/bcorrelator 250 to detect patterns indicative of boundaries between PHYpreamble fields. For example, the STF/LTF boundary detector 260 maydetect the transition from the repeating sequences a, a, . . . a to b togenerate a signal indicative of a boundary between the STF and the LTFfields. It will be noted that the STF/LTF boundary detector 260 maysimilarly detect a transition from a to −b, from b to a, a′ to b′, etc.More generally, a detector such as the STF/LTF boundary detector 260,may detect a change from a first sequence (e.g., a) to a second sequence(e.g., b) that is the complementary sequence to the first sequence. Itwill be also noted that the STF/LTF boundary detector 260 may detectmultiple transitions in a preamble and accordingly generate multiplesignals, possibly indicative of different transitions in the preamble.To take one example, the STF/LTF boundary detector 260 may generate afirst signal in response to the transition from a to b, and a secondsignal in response to the transition from b to a. The PHY preambleprocessor 92 in some embodiments may interpret the first transition as atransition from SYNC to SFD, and the second transition as a transitionfrom SFD to CES.

With continued reference to FIG. 6, the u/v correlator 258 may detectsymbol patterns defining CES symbols (e.g., u and v or u′ and v′) whichmay have lengths that are 2, 4, 8, etc. times greater than individual aand b sequences. The symbols u and v (or u′ and v′)may be comprised of2, 4, 8, etc. individual a and b sequences augmented by cover codes. Tothis end, the u/v correlator may 258 may, in some embodiments, receivecover code information from the cover code generator 254. In someembodiments, the functionality of the u/v correlator may 258 may bedistributed among the PHY preamble decoder 264, the cover code detector254, etc. Upon detecting symbol patterns u and v, the u/v correlator may258 may supply signals that indicate occurrences of u and v in thereceived signal to the channel estimator 262 for further processing.Optionally, the u/v correlator may 258 also may supply signals thatindicate occurrences of u and v in the received signal to the PHYpreamble decoder 264.

Based on the output from the cover code detector 254, STF/LTF boundarydetector 260, and possibly other components (e.g., the a/b correlator250), the PHY preamble decoder 264 may determine various operationalparameters communicated in the PHY preamble. In particular, the PHYpreamble decoder 264 may determine whether the PHY preamble specifies SCor OFDM mode, regular or low SC, determine a header rate, determine apiconet ID, etc.

By way of illustration, FIG. 7 depicts examples of cross-correlation andautocorrelation outputs that the a/b correlator 250 may generate inresponse to an example signal received via the input 252. In particular,the graph 310 corresponds to the cross-correlation with a (XCORR A), thegraph 312 corresponds to the cross-correlation with b (XCORR B), and thegraph 314 corresponds to the autocorrelation (AUTO-CORR). A plurality ofpeaks 318 in the graph 310 correspond to the locations of the sequence ain the received signal. Similarly, the plurality of peaks 320 in thegraph 320 correspond to the locations of the sequence b in the receivedsignal. A vertical line 324 generally corresponds to the STF/LTFboundary. To the left of the STF/LTF boundary, which corresponds to thetime before the STF/LTF boundary has occurred, there are a plurality ofpeaks in XCORR A that occur at intervals corresponding to the length ofa, and no peaks in XCORR B. Then, generally at the STF/LTF boundary, nopeak occurs in XCORR A, but a peak occurs in XCORR B. This pattern couldbe used, for example, to detect the STF/LTF boundary. Alternatively, theSTF/LTF boundary may be detected using the graph 314 by detecting, forexample, the falling edge of the autocorrelation “plateau” 322.

Thus, by analyzing patterns in one or more of XCORR A, XCORR B, andAUTO-CORR, the STF/LTF boundary detector 260 may detect transitionsbetween a and b sequences. Similarly, other components of the PHYpreamble processor 92 may use the one or multiple correlation outputsfrom the a/b correlator 250 to further process the received signal,e.g., to determine cover codes, to take one example.

Various example PHY preamble formats will now be described. Suchpreambles may be generated by the system of FIG. 5, for example.Similarly, such preambles may be processed by the system of FIG. 6, forexample. FIG. 8 is a diagram of one example of a PHY preamble format 350In general, the PHY preamble 350 may precede a frame header and apayload similar to the frame header 160 and the payload 162 discussedabove with reference to FIG. 4, or may be used with any other desiredformat of a data unit. The PHY preamble 350 includes an STF field 352and an LTF field 354. The STF field 352 may include several repetitionsof the same sequence a, including the last instance 356. In someembodiments, the STF field 352 may perform the function of the SYNCfield 124 (see FIG. 3) and/or the SYNC field 154 (see FIG. 4) of theprior art PHY preambles, i.e., the receiving device 14 may use therepeating sequences in the STF field 352 to detect the beginning oftransmission, synchronize the clock, etc.

Similarly, the LTF field 354 may perform the function of the CES fields128 or 158 (see FIGS. 3 and 4) of the prior art PHY preamble in at leastsome of the embodiments of the efficient PHY preamble format 350. Forexample, the LTF field 354 may include a pair (or a longer sequence) ofcomplementary CES symbols (u, v) and, in some embodiments, correspondingcyclic prefixes and/or cyclic postfixes. As indicated above, a CESsymbol may be comprised of multiple individual a and b sequencesaugmented by cover codes. In some cases, a CES symbol may have acorresponding complementary sequence. For example, if sequences a and bare complementary Golay sequences, then [a b] and [a −b] are alsocomplementary Golay sequences, and [b, a] and [b −a] are complementaryGolay sequences. It is also possible to form longer sequences byrecursively applying this rule to the pairs [a b] and [a −b], [b, a] and[b −a], etc. As used herein, the term “complementary CES symbols” refersto a pair of CES symbols that are complementary sequences such as, forexample, complementary Golay sequences.

Generally with respect to Golay sequences, it is also noted that if aand b define a pair of complementary Golay sequences, then a and −b alsodefine a pair of complementary Golay sequences. Further, an equal cyclicshift of complementary Golay sequences a and b produces a pair ofcomplementary Golay sequences a′ and b′. Still further, a pair ofcomplementary Golay sequences a″ and b″ may be generated by shiftingeach of the sequences a and b by a non-equal number of positions.

In the example illustrated in FIG. 8, a CES symbol 360 (u) is precededby a cyclic prefix 362, which is a copy of the last portion of the CESsymbol u. For the purposes of clarity, FIG. 8 and other diagrams of thepresent disclosure depict prefix and postfix relationships with arrowsdirected from a portion of a CES symbol toward the corresponding copyoutside the CES symbol. To consider one particular example, the CESsymbol 360 may be a 512-chip long Golay sequence, and the cyclic prefix362 may be a copy of the last 128 chips of the CES symbol 360. Ingeneral, the CES symbol 360 may be followed by a cyclic postfix of theCES symbol 360, by another CES symbol, by a cyclic prefix of another CESsymbol, etc. Further, it will be noted that the LTF field 354 mayinclude multiple repetitions of CES symbol patterns. At least some ofthese embodiments are discussed in more detail below.

As explained previously, the CES symbol u is comprised of complementarysequences a (also used in the STF field 352) and b, augmented by covercodes. Thus, the last portion 356 of the STF 352 is a complementarysequence corresponding to the first portion of the LTF 354, which in theembodiment of FIG. 8 is the cyclic prefix of the CES symbol 354. In atleast some embodiments, the sequences a and b are complementary Golaysequences. It will be noted that the boundary between the STF field 352and the LTF field 354 corresponds to the end of the last portion 356 ofthe STF 352 and the beginning of the cyclic prefix 362. The a/bcorrelator 250 and the STF/LTF boundary detector 260 may thus determinethe end of the STF field 352 and the beginning of the LTF field 354 bycross-correlating the received signal with one or both sequences a andb, and/or generating an auto-correlation of the received signal.

FIG. 9 is a diagram of one particular example of a PHY preambleconsistent with the efficient format discussed above with reference toFIG. 8. For the purposes of conciseness, the STF and LTF fields shall bereferred to hereinafter simply as “STF” and “LTF.” The PHY preamble 370includes a series of sequences a transmitted repeatedly with the samepolarity (+1) until the end of STF, and LTF with at least one cycle thatincludes a pair of complementary CES symbols u and v, each twice as longas the sequence a, and the corresponding cyclic prefixes and postfixesof u and v. Of course, LTF may include any suitable number of cycles.For the purposes of simplicity, however, LTF in FIG. 8 and in thesubsequent diagrams shall be illustrated with only one cycle. It will benoted that the cyclic prefix +b of the CES symbol u is associated with aspreading sequence b complementary to the spreading sequence a used withthe last portion of STF. Accordingly, the cyclic prefix +b may serveboth to reduce or eliminate ISI, and to delimit the boundary between STFand LTF. The PHY preamble 370 thus efficiently eliminates the SFD field(see FIGS. 3 and 4), and is thus shorter than the prior art preambles ofFIGS. 3 and 4. Moreover, the PHY preamble 370 may be used as a commonpreamble for both SC and OFDM modes of communication.

FIG. 10 is a diagram another example of a PHY preamble consistent withthe efficient format discussed above with reference to FIG. 8. The LTFof a PHY preamble 380 includes CES symbols u and v identical to thesymbols u and v of FIG. 9. However, LTF in the PHY preamble 380 omitsthe cyclic postfixes of the CES symbols u and v. The format illustratedin FIG. 10 may be particularly useful for frequency-domain channelestimation. The PHY preamble 380 may be also used for SC communications,although the receiver may experience some ISI in the estimated channelsdue to the absence of postfixes. As in the format of FIG. 9, thereceiver may detect the STF/LTF boundary based on the difference incorrelation output between the last symbol of STF and the first symbolof LTF.

FIG. 11 is a diagram of example of a PHY preamble. LTF of a PHY preamble390 includes at least one cycle during which complementary CES symbolsu′=[b a] and v′=[b −a] are transmitted. The CES symbol u′ is transmittedimmediately following the last period of STF (i.e., there is no cyclicprefix to u′). However, because the sequence a transmitted in the lastperiod of STF is identical to the last portion of the CES symbol u′, thelast sequence of STF advantageously serves as the cyclic prefix of theu′ (as well as the complement of the first portion b of the CES symbolu′). In this manner, the format illustrated in FIG. 11 further reducesthe length of the PHY preamble as compared to the example format of FIG.9.

FIG. 12 is a diagram of another example of a PHY preamble 400 thatincludes a series of sequences a transmitted repeatedly until the end ofSTF, and LTF with a pair of complementary CES symbols u =[a b a −b] andv =[a b −a b] and the corresponding cyclic prefixes and postfixes. Ingeneral, the length of u and v symbols can be expressed as

Length(u)=Length(v)=n Length(a)=n Length(a),  (11)

where n is a positive integer equal to or greater than two. Preferably,n is a multiple of two. In the example of FIG. 12, n is four. In thisexample, the PHY preamble 410 corresponds to a structure largely similarto the PHY preamble 370 (see FIG. 9), in that the cyclic prefix −b ofthe CES symbol u is associated with a spreading sequence b that iscomplementary to the spreading sequence a, which is used as the lastperiod of the field STF.

FIG. 13 is a diagram of another example of a PHY preamble 410. The LTFof the PHY preamble 410 includes CES symbols u and v identical to thesymbols u and v of FIG. 12. However, LTF in the PHY preamble 410 omitsthe cyclic postfixes of the CES symbols u and v. The format illustratedin FIG. 13 may be used in frequency-domain channel estimation in OFDM orSC, for example, although the receiver may experience some ISI in theestimated channels in the SC mode. As in the format of FIG. 12, thereceiver may detect the STF/LTF boundary based on the difference incorrelation output between the last symbol of STF and the first symbolof LTF.

FIG. 14 is a diagram of another example of a PHY preamble 420. The CESsymbol u′ of the PHY preamble 420 is transmitted immediately followingthe last period of STF. However, because the sequence a transmitted inthe last period of STF is identical to the last portion of the CESsymbol u′, the last sequence of STF advantageously serves as the cyclicprefix of u′ (as well as the complement of the first portion −b of theCES symbol u′). In this manner, the format illustrated in FIG. 14further reduces the length of the PHY preamble as compared to theexample preamble format 400 of FIG. 12.

From the discussion of FIGS. 9-14, it will be appreciated that a commonPHY preamble may be defined for use in SC and OFDM modes ofcommunication; that the STF/LTF boundary may be signaled usingcomplementary spreading sequences such as Golay sequences, for example;that postfixes sometimes may be omitted at a relatively small cost tothe quality of channel estimation; and that the PHY may be furthershortened by selecting the first CES symbol so that the last sequence ofthe CES symbol are identical to the sequence transmitted in the lastperiod of STF. It will be also noted that in general, CES symbols of anydesired length may be used.

FIG. 15 is a diagram of another example of a PHY preamble 430. In thePHY preamble 430, LTF includes two CES symbols u=[−b a b a] and v=[−b −a−b a]. The CES symbol v is immediately followed by its cyclic postfix,−b. Similar to the examples discussed above, STF includes a series ofrepeated sequences a. In the particular embodiment of FIG. 15, the lastperiod of STF is equal to the last period of the first CES symbol u. Thefirst symbol of the CES symbol u is −b, which is complimentary to thespreading sequence a in the last period of the STF. Thus, the lastperiod of STF serves both as a delimiter between STF and LTF and as acyclic prefix of the CES symbol u. Moreover, the last period of the CESsymbol u is equal to the last period of the symbol v, thus providing theadditional function of a cyclic prefix of the CES symbol v. From theforegoing, it will be appreciated that although the CES symbol vimmediately follows the CES symbol u which, in turn, immediately followsSTF, each of the CES symbols u and v is provided with both prefixes andpostfixes. As a result, the PHY preamble 430 is a highly efficientformat that may accommodate information sufficient for both SC and OFDMcommunication modes.

FIG. 16 is a diagram of another example of a PHY preamble 440. The PHYpreamble 440 includes CES symbols u′ and v′. In this example, the CESsymbol u′ is preceded by the cyclic prefix b transmitted at thebeginning of LTF. As compared to the PHY preamble 440 of FIG. 15, eachin the sequence of symbols of u′ is transmitted using a sequence (e.g.,a or b) complimentary to the sequence used with the respective symbol ofu while applying the same cover code to the sequence (e.g., −a in u′corresponds to −b in u, b in u′ corresponds to a in u, etc.). The CESsymbols v and v′ have the same relationship. In other words, u′ and v′are constructed by “flipping” each respective spreading sequence inevery period of u and v. Because STF in the preambles 430 and 440 is thesame, b is transmitted at the beginning of LTF to provide an STF/LTFdelimiter and a cyclic prefix for u′. As in at least some of theexamples discussed above, the PHY preamble 440 may be used for both SCand OFDM modes of operation.

FIG. 17 is a diagram of another example of a PHY preamble 450. The STFincludes a relatively short field in which the sequence b is repeatedlytransmitted after a repeated transmission of a in an earlier portion ofSTF. In a sense, several repetitions of b (in this example, two periods)serve as an explicit frame delimiter (“FD”) and, accordingly signalframe timing in a reliable manner. LTF includes CES symbols u′ and v′,with the last portion in u′ matching the sequence and the cover code inFD. As a result, the last period of FD both signals the end of STF andprovides a cyclic prefix of U′. If desired, the number of periods in FDcould be increased (i.e., there could be three or more b sequences).Referring to FIG. 16, it will be also noted that the PHY preamble 440illustrated in FIG. 16 may be considered to include FD with the lengthof 1. Thus, the boundary between STF and LTF in the preamble 440 couldbe interpreted to be the beginning of the symbol u′.

FIG. 18 is a diagram of another example of a PHY preamble 460. In theexample preamble 460, CES symbols u and v are adjacent, and u istransmitted immediately at the beginning of LTF. Similar to the casediscussed above with reference to FIG. 15, the last periods of STF and uprovide additional functions of the respective prefixes of u and v. FIG.19 is a diagram of another example of a PHY preamble 470. The PHYpreamble 470 is similar to the format of the PHY preamble 460, exceptthat the last period of LTF (the postfix of v) is omitted. As discussedabove, this format may be used in both SC and OFDM modes at somepotential cost to the quality of channel estimation.

FIG. 20 is a diagram of another example of a PHY preamble 480. The PHYpreamble 480 includes a FD at the end of STF. In this example, FDincludes two periods during which the sequence b is transmitted. Ofcourse, FD having other lengths also can be used (e.g., one period orthree or more periods). The last sequence b of FD serves as a prefix foru, and the last b sequence of u serves as a prefix for v. FIG. 21 is adiagram of another example of a PHY preamble 490. The PHY preamble 490omits the last period of LTF which the PHY preamble 480 uses to transmitthe cyclic postfix of v. FIGS. 22 and 23 are diagrams of further exampleof a PHY preambles 500, 510. The preambles 500, 510 each include acyclic prefix of the first CES symbol in the first period of LTF, and inwhich the cyclic prefix at the beginning of LTF is also a sequencecomplementary to the sequence used in the last period of STF, andtherefore serves as a reliable STF/LTF delimiter. Also, the last bsequence in u serves as a prefix for v. It will also be noted that thePHY preambles 500 and 510 of respective FIGS. 22 and 23 are similarexcept for the omission of the cyclic postfix of v in the PHY preamble510.

It will be noted that FIGS. 15-23 illustrate various embodiments inwhich four-period CES symbols u and v are efficiently used to eliminateat least some of cyclic prefixes, cyclic postfixes, and (in at leastsome embodiments) explicit SFD fields. Further, it is shown in FIGS.15-23 that the second CES symbol may be transmitted immediately afterthe first CES symbol while still eliminating ISI (i.e., because a cyclicprefix for v is provided by u). Still further, in some embodiments, thefirst CES symbol may be transmitted at the immediate beginning of LTF(i.e., following STF without intervening periods), where the lastsequence in STF provides a cyclic prefix for the first CES symbol.

Next, FIG. 24 illustrates a technique whereby the selection of a and bsequences in STF and LTF indicates different modes of transmission(e.g., an SC mode or an OFDM mode). PHY preambles 520 and 530 have thesame format, except that sequences a and b are swapped. In particular,the PHY preamble 520 corresponds to a format similar to the oneillustrated in FIG. 15, with the spreading sequence a used in STF,whereas the PHY preamble 530 has the same format as the PHY preamble520, except that the sequences a and b are swapped. The PHY preamble 520may be used for SC communications while the PHY preamble 530 may be usedfor OFDM communications. Of course, the opposite association between thepreambles 520 and 530 and PHY modes may be used instead. In one aspect,FIG. 24 illustrates a common preamble format that can be used in both SCand OFDM communications and so that a receiving device (e.g., thereceiving device 14 of FIG. 1) can determine whether the packet istransmitted via SC or OFDM by analyzing the preamble. For example, anSTF with a sequences may indicate SC mode, whereas an STF with bsequences may indicate OFDM mode.

FIG. 25 illustrates a technique of signaling SC/OFDM selection butrelies on the PHY preamble format discussed above with reference FIG.16. More specifically, PHY preambles 540 and 550 have an LTF thatincludes a cyclic prefix for u′ at the beginning of LTF, u′, v′immediately following u′, and a cyclic postfix of v. The preambles 540and 550 are the same except that the sequences a and b are swapped. TheSTF with a sequences may indicate SC mode, whereas an STF with bsequences may indicate OFDM mode. The sequence a in STF indicates the SCmode of operation, while the spreading sequence b in STF indicates OFDMmode (or vice versa). Although FIGS. 24 and 25 were discussed withrespect to encoding the parameter to indicate an SC mode versus an OFDMmode, the same technique can be used to indicate other modes orparameters.

FIG. 26 illustrates a technique whereby the selection of a and bsequences in STF indicates different modes of transmission (e.g., an SCmode or an OFDM mode). Whereas LTF in PHY preambles 560 and 570 isessentially the same, STF in the PHY preamble 560 (which may correspondto SC) uses the sequence a and STF in the PHY preamble 570 uses b (whichmay correspond to OFDM). As a result, a receiving device (e.g., thereceiving device 14) may detect the STF/LTF boundary in the OFDM modeonly after the first period of LTF. If desired, the PHY preamble 570 maybe viewed as having LTF that begins with the first period of the firstCES symbol, and in which the cyclic prefix of the first CES symbol isthe last period of STF. An STF with a sequences may indicate SC mode,whereas an STF with b sequences may indicate OFDM mode.

FIG. 27 uses the preamble format similar to the PHY preamble 480 of FIG.20, and applies a swap of the sequences a and b to SC mode or OFDM mode.The technique of FIG. 27 is similar to the technique of FIG. 25, exceptthat a different u′ is utilized. FIG. 28 illustrates another techniquewhereby the selection of a and b sequences in STF indicates differentmodes of transmission (e.g., an SC mode or an OFDM mode). FIG. 28 issimilar to the technique of FIG. 26, except that a different u′ isutilized.

As yet another approach, PHY mode selection (or selection of otheroperational parameters of the PHY layer or possibly other layers) may besignaled by including an explicit SFD field between the STF and LTFfields, and by altering various parameters of SFD. FIG. 29 is an examplePHY preamble format 620 in which a PHY mode or parameter may beindicated via cover codes in SFD, by applying particular complementarysequences a, b (e.g., complementary Golay codes) within SFD, or byvarious combinations of these techniques. For example, LTF may utilizecomplementary sequences a′ and b′, and the last period of SFD mayutilize the sequence complementary to the sequence of the first periodof LTF. Meanwhile, STF may be utilize another sequence such as a. Thus,the PHY preamble 620 may use more than a single pair of complementarysequences. Generally speaking, it is possible to use any suitablesequences in STF in all but the last period of SFD, as long as theboundary between SFD and LTF is clearly signaled by a pair ofcomplimentary sequences. Thus, STF may use one or two of the sequences aand b utilized in LTF, one or both sequences a′ and b′ corresponding tocyclically shifted respective sequences a and b, or one or several othersequences (e.g., c, d, etc.) independent of the sequences a and b, i.e.,not equal to or derived from the sequences a or b.

FIG. 30 illustrates one example technique of using SFD to indicate twoor more physical PHY modes. For ease of illustration, the framedelimiter field (FD) is illustrated in FIG. 30 as the last part of STFin each of the PHY preambles 630 and 640. To signal between SC and OFDMwithout altering u′ and v′, a pattern [b b] can be used for SC andanother pattern [−b b] can be used for OFDM. It will be noted that ineach of these two cases, the last period of FD is a sequencecomplimentary to the sequence used in the first period of LTF, thussignaling the STF/LTF boundary. In general, the FD sequence used as thelast part of the STF may include any desired number of periods, and theselection of SC or OFDM may be signaled using different cover codes. Asanother example, FIG. 31 illustrates PHY preambles 650 and 660 that useanother CES symbol u′ but otherwise are to the same as the preambles ofFIG. 30.

Next, FIG. 32 illustrates a method of indicating operational parameterssuch as SC/OFDM selection by altering the relative order of CES symbolsin LTF. As illustrated in FIG. 32, a PHY preamble 670 includes a CESsymbol u transmitted immediately before another CES symbol v. On theother hand, a PHY preamble 680 includes the CES symbol v immediatelypreceding the CES symbol u. In this embodiment, STF of the PHY preamble670 and 680 is the same. Thus, the PHY preambles 670 and 680 areidentical except for the ordering of the CES symbols in LTF. Further, uand v in this particular example are selected so as to provide cyclicprefixes and postfixes in the corresponding first and last parts of theother CES symbol. Specifically, each of the u and v symbols includes −bin the first period and a in the last period. Thus, the first part(period) of u or v may serve as a cyclic postfix of the other CES symbolu or v, and the last part of u or v may serve as a cyclic prefix of theother CES symbol u or v. In other embodiments, it is possible to usesymbols u and v that do not have this property, and a PHY preamble thatalters the ordering between u and v to signal PHY mode or otherparameters accordingly may include additional periods for cyclicprefixes/postfixes.

FIG. 33 illustrates another example of PHY preambles 690 and 700 inwhich an ordering of u and v CES symbols indicates an SC mode or an OFDMmode. However, it will be noted that the preambles 690 and 700 omit thecyclic postfix of u, and thus may not provide the same ISI protection asthe example of FIG. 33.

It will be further noted that in at least some embodiments, it may bedesirable to indicate other information in the PHY preamble. For exampleindicating a piconet ID may allow the receiving device associated with aparticular piconet to process data frames in that piconet and ignore,for example, data frames in other piconets. To this end, multiples pairsof Golay complementary sequences a_(i), b_(i) (or other suitablesequences) may be defined, and the selection of a certain pair (a_(i),b_(i)) in the STF, the LTF, or both may signal the piconet identity. Forexample, the pair a₁, b₁ may indicate piconet ID 1, the pair a₂, b₂ mayindicate piconet ID 2, etc.

Additionally or alternatively, cover codes in STF may signal piconetidentity. If desired, a single pair of Golay complementary sequences a,b may be used for all piconets in this case. For example, the cover codec₁=(1 1 1 1) may indicate piconet ID 1, the cover code c₂=(1 −1 1 −1)may indicate piconet ID 2, etc.

Moreover, combinations of a/b selections with cover codes in STF mayefficiently signal PHY modes, header rates, piconet identity, and otheroperational parameters, possibly signaling multiple parameters at thesame time. For example, each of the four-period cover codes (1 1 1 1),(1 −1, 1, −1), (−1, 1, −1, 1), (1, j, −1, −j) and (1, −j, −1, j) maysignal a particular unique selection of a piconet identity, SC or OFDMmode, header rate, etc. In PSK modulation schemes, for example, eachcover code defines a set of phase shifts. By selectively applying eachof these cover codes to the sequence a or b, a transmitting device maycommunicate even more parameters to the receiving device.

FIG. 34 illustrates a simple example of applying a length-four covercode to STF along with a particular selection of a or b sequence tosignal between SC regular, SC low rate common mode, or OFDM. The STFformat 710 uses the sequence a along with a cover code (1, 1, 1, 1) todefine an STF sequence pattern [a, a, a, a]. The STF format 720 uses thesame sequence a along with a cover code (−1, 1, −1, 1) to define an STFsequence pattern [−a, a, −a, a]. Finally, the STF format 730 uses thesequence b along with a cover code (1, 1, 1, 1) to define an STFsequence pattern [b, b, b, b]. Although any association between theformats 710-730 and operational parameters are possible, the exampleillustrated in FIG. 34 maps the format 710 to the tuple {SC, regularheader rate}, the format 720 to the tuple {SC, low header rate }, andthe format 730 to OFDM. Of course, this technique may also be applied tosignaling piconet identity, a combination of piconet identity withSC/OFDM, or other PHY layer parameters.

Referring to FIG. 35, a combination of a/b selection in STF, along witha particular SFD format, may also signal operational parameters of thePHY layer. In this example, the PHY preambles 750 and 760 may share thesame STF but may differ in their respective SFD fields. The SFD fieldsmay, for example, be of a different length or may use different covercodes or different sequences, etc. Meanwhile, the PHY preamble 770 foruse in OFDM uses a different spreading sequence in each period of STF. Areceiving device may first select between SC and OFDM by correlating theSTF field with a or b and, in the event that the STF field correlateswith a, further process the subsequent SFD field to determine whetherthe PHY preamble is associated with regular or low header rate.

FIG. 36 illustrates an approach similar to the one illustrated in FIG.35, except that the header rate in PHY preambles 780, 790, and 800 isindicated by the spreading sequence in STF. Meanwhile, SC/OFDM selectionis indicated by the SFD field. As in the examples discussed above, theSFD field can be spread using particular sequences, transmitted usingdifferent cover codes, varied in length, or otherwise altered todistinguish between various modes of operation.

Now referring to FIG. 37, a combination of cover codes in STF andvariations in the SFD field can be similarly used to indicate parameterssuch as PHY mode. In PHY preambles 810, 820, and 830, STF is spreadusing the same sequence a but the cover codes in at least one of the PHYmodes are different in STF. For the two remaining modes whose covercodes in STF are identical, variations in SFD may provide furtherdifferentiation.

Further, the technique illustrated in FIG. 38 with respect to PHYpreambles 840, 850, 860 relies on ordering of u and v in LTF as well ason a selection of spreading codes a and b in STF. Thus, the use of thesequence a in STF in combination with the ordering {u, v} may signal onePHY mode/rate configuration (e.g. SC regular). On the other hand, theuse of the same sequence with a different ordering of u and v, forexample, may signal a second PHY mode/rate configuration (e.g. OFDM).Finally, the use of the spreading sequence b in STF may signal the thirdPHY mode/rate configuration (e.g. SC low rate). It will be also notedthat for SC low rate common mode, the length of LTF may be shorter thanthe length of LTF of the PHY preamble used in SC regular (as illustratedin FIG. 39).

Referring again to FIG. 6, a preamble processor, such as the preambleprocessor 92 may generally process a received signal to detect dataframes, detect a start of an LTF field, and determine PHY parameters byanalyzing the PHY preamble using techniques such as described above. Forexample, the STF/LTF boundary detector 260 can detect the start of theLTF boundary based on detecting a change from a plurality of a sequencesto a b sequence, or a change from a plurality of b sequences to an asequence. The PHY preamble decoder 264 can determine PHY parameters suchas a modulation mode, a piconet ID, a header rate, etc., based on one ormore of 1) determining whether an a or b sequence is utilized in theSTF; 2) determining an order of u and v or u′ and v′ sequences in theLTF; and 3) determining cover codes in the STF, the LTF, and/or an SFD.

Next, FIG. 40 illustrates one example of a generator 900 that generatesa pair of complementary Golay sequences a and b in response to animpulse signal [1 0 0 . . . ] using a length-seven weight vector W suchas in (1) and a length-seven delay vector D such as in (2), (5) or (8).As illustrated in FIG. 40, the generator 900 may include an input 902,delay elements 904-910, adders/subtractors 920-934, and multipliers936-942. Each value in the weight vector W, given by (1) for example, ismapped to one of the inputs of a corresponding multiplier 936-942. Forthe weight vector given by (1), W₁=1 is assigned to the multiplier 936,W₂=1 is assigned to the multiplier 938, W₆=1 is assigned to themultiplier 940, W₇=−1 is assigned to the multiplier 942, etc. The valuesof the delay vector D given by (2), to take one example, are assigned tothe delay elements 904-910: D₁=1 is assigned to the delay element 904,D₂=2 is assigned to the delay element 906, etc. The elements of thegenerator 900 are interconnected as illustrated in FIG. 40 to generatethe Golay sequences a and b given by (3) and (4) in response to thevectors D and W considered in this example. Similarly, the generator 900generates Golay sequences given by (6) and (7) in response to the weightvector W given by (1) and the delay vector D given by (5). Although thetransmitting device 12 may include the generator 900, store the desiredvectors D and W in a memory unit, and apply the vectors D and W to thegenerator 900 to generate the sequences a and b, it is contemplated thatthe transmitting device 12 preferably stores two or more pairs ofsequences a and b in memory for quicker application in spreading bitsand/or generating PHY preambles.

On the other hand, the receiving device 14 may implement the correlator250 illustrated in FIG. 6 and again, in greater detail, in FIG. 41. Thecorrelator 250 has a structure generally similar to the structure of thegenerator 900. However, to generate a correlation output betweencomplementary Golay sequences a and b (determined by vectors D and W),the correlator 250 “flips” the adders and subtractors of the generator900 (i.e., replaces adders with subtractors, and subtractors withadders) and multiplies the output of the delay element to which D₇ isassigned by −1. In general, other designs of the correlator 250 arepossible. However, it will be appreciated that the example architectureillustrated in FIG. 41 implements the correlator as filter with impulseresponses which can be expressed as the reversal of the chip orderingwithin the sequences a and b, or a_(rev) and b_(rev), respectively.

Further, the a/b correlator 250 illustrated in FIG. 41 may beefficiently utilized in cooperation with the u/v correlator 258 (seeFIG. 6). FIG. 42 illustrates one embodiment of the u/v correlator 258that detects u/v correlation for u =[−b a b a] and v =[−b −a −b a]. Inthis example, a delay element 950 with a delay of 128 is connected tothe b correlation output 952 (see diagram 312 in FIG. 7 for one exampleof a cross-correlation output (XCORR B) between b and an input signal),a subtractor 956 is connected to the a correlation output 954, etc.Delay elements 958 and 960, and several additional adders andsubtractors provide u/v correlation. Of course, the factors in the delayelements 950, 958, and 960 may be adjusted if the sequences a and b oflengths other than 128 chips are used. The u/v correlator 258 maygenerate cross-correlation outputs 962 and 964 corresponding tocross-correlation between the received signal and sequences u and v,respectively.

It will be noted that the u/v correlator 258 efficiently uses thecorrelation output generated by the a/b correlator 250, and requiresonly several additional components to correlate sequences u or v. Itwill be further appreciated that a u/v correlator for other sequences uand v may be similarly constructed. As one example, a u/v correlator 970illustrated in FIG. 43 generates cross-correlation output between thereceived signal and sequences u =[b a −b a] and v =[−b −a −b a]. As inthe example illustrated in FIG. 42, the u/v correlator 970 efficientlyuses the output of the a/b correlator 250.

As discussed above, certain CES symbols u and v in LTF allow the PHYpreamble to efficiently communicate PHY level parameters using fewerperiods as compared to prior art PHY preambles. The following examplesillustrate further techniques of developing efficient u and v sequencesfor use in LTF. If STF is transmitted using repetitions of the sequencea, let

u₁=[c₁b c₂a c₃b c₄a]tm (12)

and let

v₁=[c₅b c₆a c₇b c₈a],  (13)

where each of c₁-c₈ is +1 or −1. To make u₁ and v₁ more efficient, use

c₄=c₈  (14)

and, preferably,

c₁=c₅.  (15)

The rest of the symbols c₂, C₃, C₅, and C₇ should be selected so as tomake u₁and v₁ complementary. It will be noted that other sequences u andv can be used in at least some of the embodiments discussed above.However, if conditions (14) and (15) are met, LTF can be made shorter atleast because adjacent sequences u and v provide each other with cyclicprefixes and/or postfixes. Further, the complementary sequences u₁ andv₁ may be efficiently used with another pair of complementary sequencesu₂ and u₂ so that a transmitting device may construct a PHY preambleusing the pair {u₁,v₁} or {u₂, v₂}, and the selection of one of thesetwo pairs of sequences may communicate one or several operationalparameters to the receiving device (e.g., SC or OFDM communication mode,header rate, etc.). In the case where STF unconditionally has multiplerepetitions of the sequence a, the second pair of CES symbols may bedefined similarly to {u₁,v₁}:

u₂=[d₁b d₂a d₃b d₄a]  (16)

v₂=[d₅b d₆a d₇b d₈a],  (17)

where each of d₁-d₈ is +1 or −1, where preferably

d₄=d₈  (18)

and, also preferably,

d₁=d₅  (19)

To enable the receiving device to distinguish between {u₁,v₁} and {u₂,v₂}, the sequences c₁c₂ . . . c₈ and d₁d₂ . . . d₈ should not be thesame.

In another embodiment, STF is transmitted using repetitions of either aor b. The pair of sequences {u₁,v₁} may then defined according to(12)-(14), and {u₂, v₂} may then be defined as:

u₂=[d₁a d₂b d₃a d₄b]  (20)

v₂=[d₅a d₆b d₇a d₈b],  (21)

where each of d₁-d₈ is +1 or −1; where, preferably, conditions (18) and(19) are also met; and where the rest of the symbols d₂, d₃, d₅, and d₇make u₂, v₂ complementary. It at least some of the cases consistent withthis approach, u₂ can be derived form u₁, and v₂ can be derived from v₁.Alternatively, u₂ can be derived form v₁, and v₂ can be derived from u₁.

To consider some specific examples, {u₁,v₁} may be defined according to(12) and (13), and {u₂, v₂} may be defined as:

u₂=m [c₂a c₃b c₄a c₁b]  (22)

v₂=m [c₆a c₇b c₈a c₅b],  (23)

where m is +1 or −1.

As another example, in which {u₁,v₁} is still provided by (12) and (13),{u₂, v₂} can be defined as:

v₂=m [c₂a c₃b c₄a c₁b]  (24)

u₂=m [c₆a c₇b c₈a c₅b],  (25)

where m is +1 or −1. It will be noted that this definition correspondsto “swapping” definitions for u₂ and v₂ provided by (22) and (23).

As yet further examples in which the definition of {u₁,v₁} is consistentwith (12) and (13), and where m is +1 or −1, {u₂, v₂} may be given by:

u₂=m [c₄a c₁b c₂a c₃b],  (26)

v₂=m [c₈a c₅b c₆a c₇b],  (27)

or

v₂=m [c₄a c₁b c₂a c₃b],  (28)

u₂=m [c₈a c₅b c₆a c₇b],  (29)

or

u₂=m [c₂a c₃b c₄a c₁b],  (30)

V₂=m [c₈a c₅b c₆a c₇b],  (31)

or

v₂=m [c₂a c₃b c₄a c₁b],  (32)

u₂=m [c₈a c₅b c₆a c₇b],  (33)

or

u₂=m [c₄a c₁b c₂a c₃b],  (34)

v₂=m [c₆a c₇b c₈a c₅b],  (35)

or

v₂=m [c₄a c₁b c₂a c₃b],  (36)

u₂=m [c₆a c₇b c₈a c₅b],  (37)

As indicated above, the use of STF patterns, SFD patterns, CES symbols,a/b sequences, etc., as well as various combinations of these parametersmay advantageously serve as an indication of one or PHY layer parametersassociated with the data frame. Moreover, transitions between patternsmay also be used to communicate PHY layer parameters or other data tothe receiving device. For example, a to −a transition between the lastperiod of SFD and the first period in CES may indicate SC, a to −btransition may indicate OFDM, etc.

Generally regarding the discussion above, it will be understood that theterms “transmitting device” and “receiving device” merely refer tooperational states of physical devices and are not intended to alwayslimit these devices to only receiving or transmitting in the respectivecommunication network. For example, the device 12 in FIG. 1 may operateas a receiver and the device 14 may operate as a transmitter at somepoint during operation.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), etc.

Although the forgoing text sets forth a detailed description of numerousdifferent embodiments, it should be understood that the scope of thepatent is defined by the words of the claims set forth at the end ofthis patent. The detailed description is to be construed as exemplaryonly and does not describe every possible embodiment because describingevery possible embodiment would be impractical, if not impossible.Numerous alternative embodiments could be implemented, using eithercurrent technology or technology developed after the filing date of thisdisclosure, which would still fall within the scope of the claims.

1. A method for generating a preamble of a data unit for transmissionvia a communication channel, the method comprising: generating a firstfield of the preamble using one of a first sequence or a secondsequence, wherein the first sequence and the second sequence arecomplementary sequences such that a sum of out-of-phase aperiodicautocorrelation coefficients of the first sequence and the secondsequence is zero; generating, using the other one of the first sequenceor the second sequence, an indicator of a start of a second field of thepreamble, the second field associated with channel estimationinformation, wherein the indicator of the start of the second fieldimmediately follows the first field; and generating the second field ofthe preamble.
 2. The method of claim 1, wherein the indicator of thestart of the second field occurs before the start of the second field.3. The method of claim 2, wherein the indicator of the start of thesecond field is a start frame delimiter (SFD)
 4. The method of claim 1,wherein the indicator of the start of the second field is included inthe second field.
 5. The method of claim 1, wherein the first sequenceand the second sequence are complementary Golay sequences.
 6. The methodof claim 1, wherein each of the sequence and the second sequence is a128-chip Golay spreading sequence associated with a weight vector W anda delay vector D, wherein W=[1 1−1 1 −1 1 −1], and D includes exactlyone of each number 1, 2, 4, 8, 16, 32, and
 64. 7. The method of claim 6,wherein the D vector is one of [1 2 4 8 16 32 64], [64 16 32 1 8 2 4],or [64 32 16 8 4 2 1].
 8. The method of claim 1, further comprisingmodulating the preamble according to a modulation scheme.
 9. The methodof claim 8, wherein the modulation scheme comprises Binary Phase-ShiftKeying (BPSK).
 10. The method of claim 9, wherein the modulation schemeis π/2 BPSK.
 11. The method of claim 1, wherein each of the firstsequence and the second sequence is a sequence of binary chips.
 12. Themethod of claim 1, wherein the first field is associated with providingtime synchronization with the data unit.
 13. The method of claim 1,wherein generating the second field of the preamble includes: generatinga first channel estimation sequence (CES) symbol including the firstsequence and the second sequence augmented by a first set of covercodes; and generating a second channel estimation sequence (CES) symbolincluding the first sequence and the second sequence augmented by asecond set of cover codes; wherein the first CES symbol and the secondCES symbol are complementary sequences.
 14. The method of claim 13,wherein the first CES symbol and the second CES symbol are complementaryGolay sequences.
 15. The method of claim 13, wherein the indicator ofthe start of the second field of the preamble serves as a cyclic prefixof the first CES symbol.
 16. The method of claim 13, further comprisinggenerating a cyclic postfix of the first CES symbol.
 17. The method ofclaim 13, wherein a last portion of the first field serves as a cyclicprefix of the first CES symbol.
 18. The method of claim 13, wherein asequence in a field different than the second field serves as a cyclicprefix of the first CES symbol.
 19. The method of claim 18, wherein afirst portion of the second CES symbol serves as a cyclic postfix of thefirst CES symbol; and wherein a last portion of the first CES symbolserves as a cyclic prefix of the second CES symbol.
 20. The method ofclaim 19, further comprising generating a cyclic postfix of the secondCES symbol.
 21. The method of claim 13, further comprising: generating acyclic prefix of the first CES symbol; generating a cyclic postfix ofthe first CES symbol; and generating a cyclic prefix of the second CESsymbol.
 22. The method of claim 21, further comprising generating acyclic postfix of the second CES symbol.
 23. The method of claim 13,wherein a length each of the first sequence and the second sequence isan integer N; and wherein a length of each of the first CES symbol andthe second CES symbol is at least 4N.
 24. The method of claim 13,wherein a beginning portion of the first CES symbol is the same as abeginning portion of the second CES symbol; wherein an ending portion ofthe first CES symbol is the same as an ending portion of the second CESsymbol; wherein generating the second field of the preamble with a firstorder of the first CES symbol and the second CES symbol in the secondfield indicates a first communication mode; and wherein generating thesecond field of the preamble with a second order of the first CES symboland the second CES symbol in the second field indicates a secondcommunication mode.
 25. The method of claim 1, wherein generating thefirst field of the preamble with the first sequence indicates a firstcommunication mode; and wherein generating the first field of thepreamble with the second sequence indicates a second communication mode.26. The method of claim 25, wherein the first communication mode is asingle carrier mode, and the second communication mode is an orthogonalfrequency division multiplexing (OFDM) mode.
 27. The method of claim 1,wherein generating the first field includes: applying a first cover codeto the first field to indicate a first value of a communicationparameter; and applying a second cover code to the first field toindicate a second value of the parameter.
 28. The method of claim 1,further comprising transmitting the preamble.
 29. A method forprocessing a preamble of a data unit received via a communicationchannel, wherein the preamble includes a first field and a second fieldassociated with channel estimation information, wherein an endingportion of the first field includes a first sequence and a beginningportion of the second field includes a second sequence complementary tothe first sequence, wherein a sum of out-of-phase aperiodicautocorrelation coefficients of the first sequence and the secondsequence is zero, the method comprising: generating at least onecorrelation signal using a received signal corresponding to thepreamble, wherein the at least one correlation signal includes at leastone of 1) a cross-correlation between the received signal and the firstsequence, 2) a cross-correlation between the received signal and thefirst sequence, and 3) an autocorrelation of the received signal;detecting a beginning of the second field based on the plurality ofcorrelation signals; and using the detection of the beginning of thesecond field to decode the second field.
 30. The method of claim 29,wherein the second field includes a first channel estimation symbol(CES) field and a second CES field, wherein each of the first CES fieldand the second CES field comprises the first sequence and the secondsequence augmented by respective cover codes, wherein the method furthercomprises: generating an additional plurality of correlation signalsusing the received signal, wherein the plurality of correlation signalsincludes a cross-correlation between the received signal and the firstCES field, and a cross-correlation between the received signal and thesecond CES field; using the additional plurality of correlation signalsto decode the second field .
 31. The method of claim 29, furthercomprising: determining a cover code in the first field; and whereindecoding the preamble comprises determining a value of a physical layerparameter based on the cover code.
 32. The method of claim 31, whereinthe parameter is one of a piconet identifier, a physical layercommunication mode, or a header rate.
 33. The method of claim 29,wherein generating at least one correlation signal includes generating aplurality of correlation signals including at least two of 1) thecross-correlation between the received signal and the first sequence, 2)the cross-correlation between the received signal and the firstsequence, and 3) the autocorrelation of the received signal.